The present invention relates to a rotational angular position detecting apparatus and in particular to an apparatus for detecting the angular position which is used for controlling an internal combustion engine of an automobile.
One of such apparatuses includes a rotor having equally spaced teeth (for example, at every 10.degree. C. A) along the periphery thereof, which is mounted on a cam shaft or a crank shaft and a rotation sensor including an electromagnetic pickup or Hall effect element facing to said rotor for generating angular position signals representative of the angular position of the crank shaft or cam shaft, having a duty ratio of the high level to the lower level being one to one for detecting a reference angular position (e.g., top dead center) of said shaft, the angular position signals being generated corresponding to tooth or teeth-removed portions on the periphery of the rotor. Such an apparatus has been disclosed in, for example, U.S. Pat. No. 4,233,592. In this apparatus, clock pulses having a repetition frequency f are up counted and down counted at a frequency which is half of the frequency f by a single up-down counter in correspondence with the tooth-removed and toothed portions, thereby detecting the reference position respectively by detecting the ratio of the previous period T(i-1) and the current period Ti which satisfies the relation Ti/T(i-1)&gt;K.
However, since the output of the up-down counter is up and down counted by using the rotation sensor which generates angular position signals in which the ratio of the high level to the lower level is one to one when the angular position signal is in the high and low levels, respectively for determining the reference position, in the above mentioned prior art apparatus, the accuracy of the fall position of the angular position signal as well as the rise-up position gives an adverse influence upon the determination of the reference position. Accordingly, it is difficult to determine the reference position at a high accuracy since manufacturing of a rotor which is excellent in accuracy of both rise-up and fall positions of the angular position signal. It is impossible to keep the ratio of the high level to the low level one to one over an entire range of the rotational number of the engine since the output signal from the electromagnetic pick-up increases as the rotational number of the internal combustion engine increases particularly in the case where the electromagnetic pick-up is used as a rotation sensor. A problem such as wrong or impossible detection of the reference position may arise in some cases.